The conventional industrial processes for the production of methanol have used a means to convert synthesis gas (a mixture of carbon monoxide, carbon dioxide, and hydrogen) into methanol. An early example of this approach is described in U.S. Pat. No. 1,569,775. This process used a mixed catalyst of chromium and manganese oxides. The process conditions were relatively extreme requiring high pressures ranging from 50 to 220 atm, and temperatures up to 450° C. The production of methanol from synthesis gas was further advanced in the early 1960s with the development of the more efficient ICI Low Pressure Methanol (LPM) process with the use of new catalysts (typically copper based) capable of operating at lower pressures and temperatures. The ICI LPM process is the most commonly used process for the production of methanol today. Today, the most widely used catalyst is a mixture of copper, zinc oxide with an alumina support, typically utilised as a fixed bed catalyst. The process is typically operated at 5-10 MPa (50-100 atm) and from 200 to 250° C.
Synthesis gas may be produced from coal, naphtha and natural gas sources. The common process is known as steam-methane reforming (SMR) and is carried out at moderate pressures of around 2-3 MPa (20-30 atm) and high temperatures (around 850° C.), over a nickel catalyst to produce syngas according to the chemical equation:CH4+H2O→CO+3H2 
This reaction is endothermic, and the heat transfer limitations place limits on the size of and the pressure in the catalytic reactors used. The reaction is carried out in the presence of an excess of steam, and is not selective due to the water gas shift reaction, during which, a significant proportion of the carbon monoxide formed is converted to carbon dioxide with the production of more hydrogen.CO+H2O→CO2+H2 
Methane is also known to undergo partial oxidation with molecular oxygen to produce syngas, as the following equation shows:2CH4+O2→2CO+4H2 
This reaction is exothermic, and the heat given off can be used in-situ to drive the SMR reaction. When the two processes are combined, it is referred to as auto thermal reforming. This reaction is also not selective, and some of the methane and/or carbon monoxide is completely oxidized to carbon dioxide.2CO+O2→2CO2 CH4+2O2→CO2+2H2O
It is inevitable that synthesis gas will always contain a finite proportion of carbon dioxide, the precise amount being dependent upon the technology used for the manufacture of the synthesis gas, and the exact reaction conditions used.
The foregoing equations illustrate that the production of synthesis gas from methane via the steam reforming route produces three moles of hydrogen gas for every mole of carbon monoxide or four moles of hydrogen for every mole of carbon dioxide. The synthesis of methanol from syngas consumes only two moles of hydrogen gas per mole of carbon monoxide; this means that there is excess hydrogen using this approach. One method that could be utilised to deal with this excess hydrogen would be to inject carbon dioxide into the methanol synthesis reactor, where it will react with the excess hydrogen to form methanol according to the equation:CO2+3H2→CH3OH+H2O
Alternatively, the hydrogen may be recovered and used elsewhere within the petrochemical complex.
The hydrogenation of carbon dioxide is said to be faster than that of carbon monoxide and according to academic literature carbon dioxide should be considered as the primary source of carbon in methanol synthesis (Lee, J. S, et. al. J. Catal. 1993, 144, 414-424). Other work has shown that the production of methanol from a synthesis gas that contains carbon dioxide, occurs mainly from the carbon dioxide component, at least in the initial stages of the reaction (Chinchen, G C et al, preprints, Am, Chem, Soc. Div, Fuel, Chem, 29(5), 178, (1984).
Due in part to global warming and climate change there is a growing interest in the methanol economy and alternative routes to methanol than the conventional ICI LPM process, which uses syngas. Growing attention is now being focused on the use of carbon dioxide from such sources as carbon capture and storage (CCS), carbon dioxide capture from flue gases or carbon dioxide waste from industrial processes such as brewing. These sources of carbon dioxide have been considered for use in combination with hydrogen obtained from water electrolysis using renewable sources of energy although in principle the hydrogen could be sourced from waste streams from conventional petrochemical processes or other sources.
One example of such a process for the conversion of carbon dioxide and hydrogen into methanol is described in Atkins, S. et. al. Energy Fuels 2009, 23, 4647-4650. In this reference, a process is described, which is based on the ICI LPM process in which conventional syngas is replaced as the feedstock by a mixture of carbon dioxide and hydrogen, which is then converted to methanol over a Cu/Zn catalyst in a fixed bed arrangement. The process was operated at relatively high pressures (1400-1800 psi) and temperatures (up to 260° C.). The maximum % conversion of carbon dioxide observed was less than 15 mole % and the reaction was very unselective when compared with normal methanol production. Both hydrocarbons and carbon monoxide were detected in the effluent from the main reactor.
Recent attempts to prepare methanol from carbon dioxide have suggested that the simple hydrogenation route is not facile and there are significant problems with the reactivity of carbon dioxide towards hydrogen in the synthesis of methanol (Mitsui Chemicals Inc.: “A New Leading Process for CO2 to methanol”. New Energy and Fuel, Aug. 29, 2008.
A further example is provided in published international application WO 2010/011504 A3. In this document a process is described in which all of the raw materials and energy for methanol production are obtained from a geothermal source.
These emerging processes for the utilisation of carbon dioxide/hydrogen feeds are not, as yet, optimized processes and have significant problems and challenges that are typically associated with such catalytic processes.
A major challenge in many catalytic processes is indentifying operating conditions, which ensure optimum utilization of the catalysts and/or process conditions and which operate with satisfactory selectivities. Catalysts have a useful life and must eventually be replaced or reconditioned in order to keep the process operating at the optimum conditions. Not all catalysts can be reconditioned, and copper/zinc catalysts in particular cannot be reconditioned and the poisoning and sintering of the catalysts is irreversible. The copper/zinc catalysts that are need for methanol synthesis fall into this category. The initial conditioning of the catalyst, the start-up conditions and ongoing operating conditions all have an impact on overall catalyst performance. Through any given cycle the catalyst activity will diminish and this is often compensated by changing the process to conditions that are even harsher on the catalysts resulting in accelerated catalyst deactivation. There is a typical tradeoff between the costs of catalyst replacement compared to the increased running costs to maintain activity.
This is a particular problem for catalysts used in processes for the conversion of carbon dioxide/hydrogen where harsh conditions may be required and catalyst life is shortened as a consequence.